1. Field of the Invention
The present invention relates to an image forming apparatus for forming a document having formed thereon a background pattern which includes a copy prohibiting pattern for preventing an unfair copying.
2. Description of the Related Art
Conventionally, various techniques have been proposed for preventing or repressing copying or reproduction of a confidential document or a use of a copy of such document, which includes, for example, company's information to be kept secret. Examples of the foregoing conventional techniques include a technique for embedding into a document image a background pattern made up of a background portion formed with a screen ruling and a latent image portion formed with a screen ruling which is higher (higher dot density) than that of the background portion (hereinafter referred to as background pattern print technique). When copying a document image formed by the foregoing background pattern print technique, an outlined pattern such as “copy prohibited” appears on the copy in an area corresponding to the latent image portion of the document image.
Japanese Patent Unexamined Publication No. 2005-115874 (Published on Apr. 28, 2005) discloses such background pattern print technique wherein an image to be appeared on a copy when copying a document can be set as desired by inputting an image file for an image which represents an arbitrary shape such as a logo.
Another well known conventional technique for preventing an unfair copying is disclosed, for example, in Japanese Patent Unexamined Publication No. 2004-274092 (Published on Sep. 30, 2004; Corresponding US Patent Application Publication No. 2005/0078331 (Published on Apr. 14, 2005)), wherein a special dot pattern embedded into a background image of the image data obtained by reading a document image is detected, and as a result of comparing the special dot pattern as detected with a special dot pattern as stored in memory, if the special dot pattern matches the one stored in the memory, an image forming operation of the document image is prohibited (hereinafter, this technique is referred to as a copy guard technique).
In recent years, a technique which combines the foregoing background pattern print technique with the copy guard technique (hereinafter, referred to as a hybrid background pattern print technique) has been proposed, wherein a special dot pattern for use in the copy guard function is formed in a background portion. With this structure, when an attempt is made to copy a document prepared by using the foregoing hybrid background pattern print technique by a copying machine which is provided with the function of detecting a special dot pattern, it is possible to prevent an unfair copying of the document. Furthermore, even when an attempt is made to copy the document prepared by using the hybrid background pattern print technique by a copying machine which is not provided with the function of detecting a special dot pattern, since the pattern would appear on a copy, it is possible to expect such effect that an operator refrains from performing an unfair copying or a use of the copy.
When adopting the foregoing hybrid background pattern print technique, it is possible to realize a more desirable image forming apparatus by arranging a pattern to be appeared on a copy can be set as desired. However, the following points should be taken into consideration.
That is, as described above, the background portion is formed with a screen ruling and includes dots which can be resolved by a reading operation of a normal (having an average reading accuracy) image forming apparatus. On the other hand, the latent image portion is formed with a screen ruling which has a screen ruling higher than the screen ruling of the background portion and includes fine dots which cannot be resolved by a reading operation performed by a normal image forming apparatus.
Since the special dot pattern needs to be resolved by an image forming apparatus, it is necessary to form the special dot pattern on the background portion. However, if an area of the background portion in the document is relatively small, the special dot pattern cannot be formed under stable conditions.
Specifically, in an image forming apparatus wherein a special dot pattern presents a certain pattern, if an area of the background portion is relatively small, the pattern may be lacked partially. Further, some image forming apparatuses are arranged such that a predetermined number of special dot patterns are formed in a background portion of a document, and a copy guard function becomes effective only after detecting a predetermined number of dot patterns out of the dot patterns formed on the document. With this structure, if an area of the background portion is relatively small, there may not have enough space for forming the predetermined number of dot patterns.